Image reconstruction methods in which a three-dimensional image dataset is determined from a multiplicity of two-dimensional projection images that were recorded in different recording geometries, i.e. from different projection directions, are well known in the prior art. Iterative reconstruction methods or filtered back-projection methods can be used, for example. Problems always occur when the recording volume (i.e. the target region) moves, as in the case of target regions surrounding the heart, for example.
After originally using motion estimation methods that already include in principle the assumption of periodicity in the case of target regions of a body that are subject to periodic motion, satisfactory reconstruction quality was still found to be lacking. This is explained by the fact that the various instances of periodic motion nonetheless exhibit small differences, which have a negative influence on the reconstruction quality.
Surprisingly, it was found that the reconstruction quality can be significantly improved using non-periodic motion models, i.e. it is taken as a starting point that a non-periodic motion will be estimated, wherein said non-periodic motion can be used in a dynamic reconstruction algorithm (such as those that are already well known) in order to obtain a significantly improved reconstruction image, i.e. an artifact-free three-dimensional image dataset of higher quality. A method of the type cited in the introduction is disclosed in EP 2 242 023 A1, for example. This describes a method for the motion-compensated reconstruction of a three-dimensional definitive reconstruction dataset of a recording volume (which moved during a recording period) from two-dimensional projection images, using a dynamic and in particular analytical reconstruction algorithm, wherein for the purpose of determining the in particular location-dependent non-periodic motion during the recording time, an initial parameter set describing a possible motion in at least one motion model, in which the time dependency is described by the recording time, is first defined as a current parameter set, whereupon, in the context of an optimization method relating to the parameter set, a current reconstruction dataset is determined by means of the dynamic reconstruction algorithm with reference to the possible motion described by the current parameter set and is evaluated on the basis of a target function which comprises an evaluation measure, such that when a convergence criterion for the target function is finally satisfied, the optimization method can be terminated and the current reconstruction dataset can be used as the definitive reconstruction dataset. The definitive reconstruction dataset, which was already reconstructed during the optimization method, therefore corresponds to the three-dimensional image dataset of the present invention.
The method disclosed in EP 2 242 023 A1 therefore uses a parameterizable non-periodic motion model, which is therefore not defined by phases of a periodic motion but by the current time, such that recording times or the recording instant can be used immediately, without requiring a periodic motion as a basis. Various embodiments are conceivable in respect of the target function, wherein both comparison with a three-dimensional reference dataset and comparison with the recorded projection images are possible, wherein forward-projection images can be determined from the current reconstruction dataset using dynamic forward projection, and their similarity to the actual recorded projection images can be evaluated. As mentioned above, the overall result is a significant improvement in quality of the three-dimensional image dataset of the target region. Optimal image quality is achieved when the generated static three-dimensional image correlates to a heart phase in which the heart is almost at rest, usually the end-diastolic phase, and also in the systolic rest phase in the case of rapid heartbeats. A non-periodic approach to the motion makes it possible at least significantly to reduce those serious impairments to the image quality that are caused by heart phases involving pronounced motion, e.g. the systolic contraction or the early diastolic dilatation.
The resulting three-dimensional image datasets, which can show e.g. the coronary arteries and the heart, are an extremely useful tool, e.g. in the planning of minimally invasive interventions, in particular using a catheter, but also for image monitoring during an intervention, when the three-dimensional image dataset is superimposed by fluoroscopy images. However, there is also considerable demand in general for four-dimensional information in this context, i.e. for a moving representation of a target region, in particular the heart region, since such four-dimensional information is useful for functional analyses and dynamic overlapping. In interventions for clearing a chronic total occlusion, for example, such moving superimpositions of images therefore provide an extremely useful aid for the navigation of a catheter through the occluded part in order to clear the occlusion by means of ablation, for example, and reduce the risk of harming or even rupturing the vessel.
Previously disclosed are merely methods in which dynamic information is generated by an electrocardiogram gating for a corresponding heart phase. The image result of the ECG gating is post-processed, e.g. by compensating for the residual motion, wherein a 4D animation of a heartbeat is provided by separate three-dimensional reconstruction of various heart phases. Due to the problems cited above in respect of the assumption of a periodic motion, and the fact that a separate three-dimensional image dataset must be specified at considerable computing cost for each heart phase, the resulting pictures are of extremely poor quality and suffer from all manner of artifacts and other quality deficiencies, and are therefore very difficult to read.
The article “High-quality 3-D coronary artery imaging on an interventional C-arm x-ray system” by Eberhard Hansis et al., Med. Phys. 37 (4), April 2010, pages 1601 to 1609, concerns the projection-based motion compensation for the reconstruction of coronary arteries in a single heart phase using a gating.
The article “ECG-Gated Interventional Cardiac Reconstruction for Non-periodic Motion” by Rohkohl et al., MICCAI 2010, Part I, LNCS 6361, pages 151 to 158, likewise describes an electrocardiogram-gated reconstruction algorithm, in which a weighting factor is used for the images of a specific heart phase. The result is a three-dimensional image dataset in a specific heart phase.
The article “Reconstruction of Coronary Arteries From a Single Rotational X-Ray Projection Sequence” by Christophe Blondel et al., IEEE TRANSACTIONS ON MEDICAL IMAGING, Vol. 25, No. 5, May 2006, pages 653 to 663, discloses a reconstruction of coronary arteries from a single projection sequence, wherein an estimation of the motion of the coronary arteries is carried out. The tomographic reconstruction of the coronary arteries includes motion compensation and is three-dimensional.